Generalize input support

[ttj]

There are currently restrictions for inputs as they are expressions plus intervals.

For example, the following does not work:

\dot{x} = u \dot{y} = v \dot{x} = u /\ \dot{y} = u let u \in [-1,1]

The semantics interpret u as really having the same value at a given point in time, so \dot{x} and \dot{y} are equal.

Fixes:

1. Add interval output support for simulator for now

2. Eventually generalize notion of inputs with inherritance (probably) for variables, e.g., InputVariable extends Variable and StateVariable extends variable